My first two children are 13 months apart. I couldn't really do much with a 13 month old....except make a big deal that she was a big sister. She really could care less...and in essence they really just grew up together and I don't really think my oldest ever really remembered a time she was an only child.
For a 20 month old...they are probably a lot more aware...but still I don't think aware enough really ....that special gifts etc. are necessary. I wouldn't make it a big deal, other than, "look who we have brought into our family". You might want to make it seem completely normal,and to be expected, that you as the parents are adding more children to your family.
I personally never wanted the "second" or "third" child to be "unwanted" by the other children...so I always just made it seem like this is what us as the parents' wanted and we are adding to our family. I didn't even say things like, "I'm sorry I won't have as much time with you" or things of that nature. I DID feel guilty during the first month of my second daughters life, because I felt that I couldn't give enough time to my first daughter. I questioned weither I could love another child the same as my first. BUT, I learned quickly, that you can love all your children the same. I HAVE NO FAVORITES among my 3...it's amazing how you can spred your love evenly over your children. I don't use the word "split" your love...because I honestly feel the same amount for each of them.
I would tell myself that, this is the way it has to be, in families with more than one child. Just make sure you make time for your son when the baby is napping...and if he starts to question time away from him given to the baby, your answer should be "I did the same thing for you when you were a baby", now it's this baby's turn. I still love you, but the new baby has certain needs that I have to take care of right now.
For some reason, the "I did the same for you, when you were a baby"...really calmed and reassured my older two when my third child was born. They understood, and I think deep down inside felt that .." Well, I got the same treatement when I was that age, now it's her turn". They would ask me all the time, like "did I do that when I was a baby??" ...and they enjoyed knowing that they were very similar and got the same attention.
I think I went a little overboard with my advice. But I basically just think, that there isn't that much preparation that needs to be done with a 20 month old before the baby is born. Just his understanding that another baby will be added to the family. (not a new baby- that could replace his "newness", just another one)Get him the tshirt and make him feel like the big brother.( but I wouldn't give him gifts, that is just kinda like a bribe and validates whatever feelings he may have about a new sibling..like you guys are sorry or something - it shouldn't be an issue and a new baby being born should be treated like something completely normal needing no apology or bribe)
I don't think he will completely understand what it means to have a younger child at home....so in that way I think this situation is perfect. He, like my oldest daughter, probably won't remember much about the time before he had a sibling. All three of mine are awsome together (oh, trust me, they have their moments NOW), but when they were younger, it was never a question of a new kid being born, just another person to add to our happy family. Kids pretty much follow your reactions to things, especially at this young age. If you promote, big happy family, and adding another sibling to your family, and be excited for the baby and let him be a part like maybe picking some toys out for the baby, he will feel like it is completely normal and be excited for the prospect of a playmate.
Hope it helps...Hang in there....your first couple years will be a little difficult with two...but it pays off in the end. My oldest two are the closest, because of their age. They are sometimes just like twins.